From: Colin Ian King Date: Wed, 9 Apr 2025 15:55:10 +0000 (+0100) Subject: select: do_pollfd: add unlikely branch hint return path X-Git-Tag: v6.16-rc1~223^2~21 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=5730609ffd7e558e1e3305d0c6839044e8f6591b;p=thirdparty%2Fkernel%2Flinux.git select: do_pollfd: add unlikely branch hint return path Adding an unlikely() hint on the fd < 0 comparison return path improves run-time performance of the poll() system call. gcov based coverage analysis based on running stress-ng and a kernel build shows that this path return path is highly unlikely. Benchmarking on an Debian based Intel(R) Core(TM) Ultra 9 285K with a 6.15-rc1 kernel and a poll of 1024 file descriptors with zero timeout shows an call reduction from 32818 ns down to 32635 ns, which is a ~0.5% performance improvement. Results based on running 25 tests with turbo disabled (to reduce clock freq turbo changes), with 30 second run per test and comparing the number of poll() calls per second. The % standard deviation of the 25 tests was 0.08%, so results are reliable. Signed-off-by: Colin Ian King Link: https://lore.kernel.org/20250409155510.577490-1-colin.i.king@gmail.com Signed-off-by: Christian Brauner --- diff --git a/fs/select.c b/fs/select.c index 7da531b1cf6be..0eaf3522abe9a 100644 --- a/fs/select.c +++ b/fs/select.c @@ -857,7 +857,7 @@ static inline __poll_t do_pollfd(struct pollfd *pollfd, poll_table *pwait, int fd = pollfd->fd; __poll_t mask, filter; - if (fd < 0) + if (unlikely(fd < 0)) return 0; CLASS(fd, f)(fd);